Problem: Which of these numbers is composite? ${11,\ 31,\ 67,\ 79,\ 81}$
A composite number is a number that has more than two factors (including 1 and itself). 11, 31, 67, and 79 each have only two factors. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 81 is the composite number.